1. Field of the Invention
The present invention relates to the preparation of polyurethanes or polyurethane-polyureas. More particularly the invention relates to the use of certain monocyclic amidines as catalysts for the urethane/urea reactions.
2. Description of the Prior Art
The preparation of crosslinked plastic substances according to the isocyanate polyaddition method is known. According to German Application No. 11 96 864 (U.S. Pat. No. 3,099,516), hydroxyl group-containing compounds and polyisocyanates are reacted for this purpose in the presence of blowing agents and catalysts. With suitable selection of the hydroxyl group-containing polyesters, polyethers, polyether esters, polyester amides and others, and organic polyisocyanates, as well as by the additional use of chain extenders, such as glycols or diamines, flexible as well as rigid polyurethane elastomers, as well as all intermediate modifications, may be prepared according to this method.
One method for the preparation of cellular molded parts with a closed surface layer of polyurethane-polyurea elastomers according to the principle of reaction injection molding is described in German Published Application No. 26 22 951 (British Pat. No. 1,534,258). The elastomer systems basically consist of organic polyisocyanates, polyols, active aromatic di- and/or polyamines which may be substituted by alkyl groups in the ortho position to the amino group and catalysts for the reaction between hydroxyl and isocyanate groups. Such systems have cream times down to less than one second. The transition from liquid into the solid phase takes place nearly instantaneously resulting in the liquid reaction mixture solidifying along the walls of the mold.
According to German Patent Application No. P 29 40 738.9, polyurethane-polyurea formulations with somewhat reduced reactivity, and thereby flowability, are obtained if 3,3',5,5'-tetra-alkyl-substituted 4,4'-diamino diphenylmethanes are used as aromatic diamines in which the alkyl radicals are the same or different and stand for a methyl, ethyl, isopropyl, secondary or tertiary butyl radical, with at least one of these substituents having to be an isopropyl or secondary butyl radical.
Further known is the use of amidines, mono- and bicyclic amidines as polyurethane catalysts. According to information in German Published Application No. 1,950,262 (Great Britian No. 1,287,150), amidines, for example, N'-cyclohexyl-N,N-dimethylformamide, are used as catalysts for the preparation of polyurethane foams from aliphatic diisocyanates. The same published application described monocyclic amidines, for instance, 3-butyl-3,4,5,6-tetrahydropyrimidine. Commonly used polyurethane catalysts are also substituted imidazoles and dialkylamino pyridines. Bicyclic amidines as urethane catalysts are described in German Published Application No. 1,745,418 (U.S. Pat. No. 3,769,244) where the use of metal organic compounds is also mentioned.
Amidines frequently have the drawback that they are easily decomposed by water (Methods of Organic Chemistry, Houben-Weyl, Vol XVI/1, page 940, 4th Edition, G. Thieme, publishers Stuttgart, 1957). This largely prevents their use in water-containing systems. However, even the small amounts of water present in polyols normally suffice for greatly reducing the catalytic effects of free amidines within a short period of time. It is therefore not possible to store amidine-containing polyol mixtures.
Moreover, the use of free amidines is not only greatly restricted by water but also by halogenated hydrocarbons. These quickly react by quaternising, resulting in a loss in catalytic effect of the amidines.
In order to eliminate these drawbacks, amidine-metal complexes are described in German Published Application No. 24 34 185 (U.S. Pat. No. 4,006,124). As illustrated in the German Application, the complexes must be prepared in a separate process step prior to being added to the polyol since the addition of the individual components to the polyol brings about considerably poor results.
The purpose of this invention was the development of catalysts facilitating a pronounced acceleration of the polyaddition reaction of hydroxyl group-containing compounds with isocyanates and which overcome the above-mentioned drawbacks.